704 research outputs found
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
A Possible Hidden Population of Spherical Planetary Nebulae
We argue that relative to non-spherical planetary nebulae (PNs), spherical
PNs are about an order of magnitude less likely to be detected, at distances of
several kiloparsecs. Noting the structure similarity of halos around
non-spherical PNs to that of observed spherical PNs, we assume that most
unobserved spherical PNs are also similar in structure to the spherical halos
around non-spherical PNs. The fraction of non-spherical PNs with detected
spherical halos around them, taken from a recent study, leads us to the claim
of a large (relative to that of non-spherical PNs) hidden population of
spherical PNs in the visible band. Building a toy model for the luminosity
evolution of PNs, we show that the claimed detection fraction of spherical PNs
based on halos around non-spherical PNs, is compatible with observational
sensitivities. We use this result to update earlier studies on the different PN
shaping routes in the binary model. We estimate that ~30% of all PNs are
spherical, namely, their progenitors did not interact with any binary
companion. This fraction is to be compared with the ~3% fraction of observed
spherical PNs among all observed PNs. From all PNs, ~15% owe their moderate
elliptical shape to the interaction of their progenitors with planets, while
\~55% of all PNs owe their elliptical or bipolar shapes to the interaction of
their progenitors with stellar companions.Comment: AJ, in pres
Outcomes of Early Endoscopic Realignment Versus Suprapubic Cystostomy and Delayed Urethroplasty for Pelvic Fracture-related Posterior Urethral Injuries : A Systematic Review
Peer reviewedPostprin
The "Twin Jet" Planetary Nebula M2-9
We present a model for the structure, temporal behavior, and evolutionary
status of the bipolar nebula M2-9. According to this model the system consists
of an AGB or post-AGB star and a hot white dwarf companion, with an orbital
period of about 120 years. The white dwarf has undergone a symbiotic nova
eruption about 1200 years ago, followed by a supersoft x-ray source phase. The
positional shift of the bright knots in the inner nebular lobes is explained in
terms of a revolving ionizing source. We show that the interaction between the
slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a
path for the ionizing radiation in one direction, while the radiation is
attenuated in others. This results in the mirror-symmetric (as opposed to the
more common point-symmetric) shift in the knots. We show that M2-9 provides an
important evolutionary link among planetary nebulae with binary central stars,
symbiotic systems, and supersoft x-ray sources.Comment: 13 pages + 2 figures. Submitted to Ap
On the Luminosities and Temperatures of Extended X-ray Emission from Planetary Nebulae
We examine mechanisms that may explain the luminosities and relatively low
temperatures of extended X-ray emission in planetary nebulae. By building a
simple flow structure for the wind from the central star during the proto, and
early, planetary nebulae phase, we estimate the temperature of the X-ray
emitting gas and its total X-ray luminosity. We conclude that in order to
account for the X-ray temperature and luminosity, both the evolution of the
wind from the central star and the adiabatic cooling of the post-shocked wind's
material must be considered. The X-ray emitting gas results mainly from shocked
wind segments that were expelled during the early planetary nebulae phase, when
the wind speed was moderate. Alternatively, the X-ray emitting gas may result
from a collimated fast wind blown by a companion to the central star. Heat
conduction and mixing between hot and cool regions are likely to occur in some
cases and may determine the detailed X-ray morphology of a nebula, but are not
required to explain the basic properties of the X-ray emitting gas.Comment: ApJ, submitted; 16 page
Magnetic Flares on Asymptotic Giant Branch Stars
We investigate the consequences of magnetic flares on the surface of
asymptotic giant branch (AGB) and similar stars. In contrast to the solar wind,
in the winds of AGB stars the gas cooling time is much shorter than the outflow
time. As a result, we predict that energetic flaring will not inhibit, and may
even enhance, dust formation around AGB stars. If magnetic flares do occur
around such stars, we expect some AGB stars to exhibit X-ray emission; indeed
certain systems including AGB stars, such as Mira, have been detected in
X-rays. However, in these cases, it is difficult to distinguish between
potential AGB star X-ray emission and, e.g., X-ray emission from the vicinity
of a binary companion. Analysis of an archival ROSAT X-ray spectrum of the Mira
system suggests an intrinsic X-ray luminosity 2x10^{29} erg/sec and temperature
10^7 K. These modeling results suggest that magnetic activity, either on the
AGB star (Mira A) or on its nearby companion (Mira B), is the source of the
X-rays, but do not rule out the possibility that the X-rays are generated by an
accretion disk around Mira B.Comment: ApJ, Accepted; revised version of astro-ph/020923
An impairment in sniffing contributes to the olfactory impairment in Parkinson's disease
Although the presence of an olfactory impairment in Parkinson's disease (PD) has been recognized for 25 years, its cause remains unclear. Here we suggest a contributing factor to this impairment, namely, that PD impairs active sniffing of odorants. We tested 10 men and 10 women with clinically typical PD, and 20 age- and gender-matched healthy controls, in four olfactory tasks: (i) the University of Pennsylvania smell identification test; (ii and iii) detection threshold tests for the odorants vanillin and propionic acid; and (iv) a two-alternative forced-choice detection paradigm during which sniff parameters (airflow peak rate, mean rate, volume, and duration) were recorded with a pneomatotachograph-coupled spirometer. An additional experiment tested the effect of intentionally increasing sniff vigor on olfactory performance in 20 additional patients. PD patients were significantly impaired in olfactory identification (P < 0.0001) and detection (P < 0.007). As predicted, PD patients were also significantly impaired at sniffing, demonstrating significantly reduced sniff airflow rate (P < 0.01) and volume (P < 0.002). Furthermore, a patient's ability to sniff predicted his or her performance on olfactory tasks, i.e., the more poorly patients sniffed, the worse their performance on olfaction tests (P < 0.009). Finally, increasing sniff vigor improved olfactory performance in those patients whose baseline performance had been poorest (P < 0.05). These findings implicate a sniffing impairment as a component of the olfactory impairment in PD and further depict sniffing as an important component of human olfaction
Welfare and Revenue Guarantees for Competitive Bundling Equilibrium
We study equilibria of markets with heterogeneous indivisible goods and
consumers with combinatorial preferences. It is well known that a
competitive equilibrium is not guaranteed to exist when valuations are not
gross substitutes. Given the widespread use of bundling in real-life markets,
we study its role as a stabilizing and coordinating device by considering the
notion of \emph{competitive bundling equilibrium}: a competitive equilibrium
over the market induced by partitioning the goods for sale into fixed bundles.
Compared to other equilibrium concepts involving bundles, this notion has the
advantage of simulatneous succinctness ( prices) and market clearance.
Our first set of results concern welfare guarantees. We show that in markets
where consumers care only about the number of goods they receive (known as
multi-unit or homogeneous markets), even in the presence of complementarities,
there always exists a competitive bundling equilibrium that guarantees a
logarithmic fraction of the optimal welfare, and this guarantee is tight. We
also establish non-trivial welfare guarantees for general markets, two-consumer
markets, and markets where the consumer valuations are additive up to a fixed
budget (budget-additive).
Our second set of results concern revenue guarantees. Motivated by the fact
that the revenue extracted in a standard competitive equilibrium may be zero
(even with simple unit-demand consumers), we show that for natural subclasses
of gross substitutes valuations, there always exists a competitive bundling
equilibrium that extracts a logarithmic fraction of the optimal welfare, and
this guarantee is tight. The notion of competitive bundling equilibrium can
thus be useful even in markets which possess a standard competitive
equilibrium
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